This invention concerns fiber reinforced compositions and articles and methods for producing such compositions and articles.
In particular, the present invention provides an apparatus and a method of prepregging reinforcing materials such as fibers with resinous or polymeric materials, especially with thermoplastic resin or polymer compositions, and the prepregs produced by such apparatus or method. The invention further provides a method of using such prepregs to form articles having highly desirable properties and to the articles produced thereby.
Reinforced thermoplastic and thermoset materials have wide application in, for example, the aerospace, automotive, industrial/chemical, and sporting goods industries. Thermosetting resins are impregnated into the reinforcing material before curing, while the resinous materials are low in viscosity. Thermoplastic compositions, in contrast, are more difficult to impregnate into the reinforcing material because of comparatively higher viscosities. On the other hand, thermoplastic compositions offer a number of benefits over thermosetting compositions. For example, thermoplastic prepregs are easier to fabricate into articles. Another advantage is that thermoplastic articles formed from such prepregs may be recycled. In addition, a wide variety of properties may be achieved by proper selection of the thermoplastic matrix.
Fiber-reinforced plastic materials may be manufactured by first impregnating the fiber reinforcement with resin to form a prepreg, then consolidating two or more prepregs into a laminate, optionally with additional forming steps. Consolidation is typically necessary to remove voids that result from the inability of the resin to fully displace air from the fiber bundle, tow, or roving during the processes that have been used to impregnate the fibers with resin. The individually impregnated roving yams, tows, plies, or layers of prepregs are usually consolidated by heat and pressure, or with heat and vacuum as by vacuum-bag molding and compacting in an autoclave. The consolidation step has generally required the application of very high pressures or vacuums at high temperatures and for relatively long times. Alternatively, the prepreg fiber bundle is chopped or pelletized and then used in a molding or extrusion process, with or without other polymeric materials or reinforcements, to produce molded or extruded articles.
In the past, a thermoplastic composition has typically been heated, slurried, commingled, or diluted with solvents, plasticizers, or other low molecular weight materials in order to reduce the viscosity of the composition before it is used to impregnate the reinforcing material. These methods have suffered from serious drawbacks. In the case of using solvent to reduce viscosity, the solvent must be driven off after the impregnation step, resulting in an additional step in the process as well as unwanted emissions. Moreover, the desired thermoplastic composition may be insoluble in or incompatible the desired diluent.
In the case of heating the thermoplastic matrix composition to a temperature at which its viscosity is low enough for satisfactory impregnation of the fiber, the dwell time of the resin in the heated zone may result in degradation of the resin with attendant decrease in the desired mechanical properties. Furthermore, the molecular weight of the resin may need to be kept lower than would be desired for properties of the ultimate product in order to facilitate the impregnation step. Finally, as noted above, known processes for impregnating thermoplastic resin into reinforcing materials have required lengthy consolidation of the prepreg materials at high temperatures and pressures in order to develop the best physical strength and other properties and to minimize or eliminate outgassing during the consolidation or in later steps, e.g., finishing processes. Outgassing during consolidation results in voids within the composite that can cause microcracking or premature delamination that may adversely affect mechanical properties; outgassing during coating steps tends to cause pinholling or popping in the substrate or coating, resulting in an undesirably rough and blemished surfaces or finishes.
Cogswell et al., U.S. Pat. Nos. 5,213,889; 5,019,450; 4,559,262; and 4,549,926 (all of which are incorporated herein by reference), teach that impregnation of fibers with thermoplastic polymers requires (1) a relatively low molecular weight polymer to give sufficiently low melt viscosity (less than 30 Ns/m2; Table 1 showing increasingly poor fiber wetting as the 30 Ns/m2 limit is approached), (2) having within the molten polymer bath a spreader with an external heat input to heat the polymer in the immediate vicinity of the spreader to a relatively high temperature, (3) slow rates of throughput (Table 2 showing significantly decreased fiber wetting at 60 cm/min, as compared to 36 cm/min), and (4) long dwell times of the fibers in the molten polymer bath (dwell time shown in Examples 1 and 5 to be 30 seconds). Conventional grades of thermoplastic material (i.e., having the molecular weights generally using in forming articles from thermoplastic materials) cannot satisfactorily wet out fibers in the Cogswell process, as is shown by patentee""s Example 4.
The present invention provides the ability to form prepregs using higher molecular weight polymers in order to achieve the highest possible strength and to optimize other physical properties in articles formed therefrom. In addition, the present inventive process is lower cost and more efficient than previous processes because the present process allows a faster throughput rate and shorter dwell time in the molten polymer bath, while at the same time producing a prepreg in which the fibers are substantially fully wet out.
In another previously known process, Cogswell et al., U.S. Pat. Nos. 4,783,349; 4,735,828; and 4,624.886 (all of which are incorporated herein by reference), use a low molecular weight plasticizer to reduce the melt viscosity of a thermoplastic impregnating resin. The filaments of the fiber bundle are pre-wetted with the plasticizer before entering the molten polymer bath, which may include a further metal plasticizer. The plasticizer is removed by volatilization when it is not desired in the final product. Thus, production of materials with high strength and other physical properties for which plasticization is undesirable requires not only the pre-wetting step but also a step of evaporating or otherwise extracting the plasticizer following impregnation. The process is also illustrated for fabrics woven of reinforcing fibers, and patentees teach that the process does not require significant mechanical work input in the form of tensioning the fiber. Cogswell et al., in U.S. Pat. No. 4,541,884 (which is incorporated herein by reference), teach a process in which a plasticizer is incorporated in the molten thermoplastic polymer bath to decrease the viscosity of the molten bath. The plasticizer is volatilized from the prepreg in a further process step.
It is undesirable to plasticize the polymer in many applications, for example when higher tensile strength is important. Moreover, the extra steps of impregnating a tow or fiber bundle with a plasticizer and volatilizing or extracting the plasticizer after the impregnation step add expense and make the process cumbersome.
Cochran et al., U.S. Pat. No. 5,236,646 (which is incorporated herein by reference), disclose that a process using vacuum of up to about 28 inches of mercury below atmospheric pressure and temperatures above the melting point of the resin requires a shorter time for consolidation as compared to a process that uses high consolidation pressures of from about 100 to 300 psi. However, the consolidation step still requires a dwell time under vacuum of up to sixty minutes or more.
Because the length of time typically required to properly consolidate the prepreg plies determines the production rate for the part, it would be desirable to achieve the best consolidation in the shortest amount of time. Moreover, lower consolidation pressures or temperatures and shorter consolidation times will result in a less expensive production process due to lowered consumption of energy per piece for molding and other manufacturing benefits.
The present invention provides a new process for preparing prepregs, the novel prepregs produced by such a process, and articles of reinforced materials that offers significant advantages over the processes described above. In a method according the present invention, the reinforcing material is heated before being impregnated with the resinous or polymeric matrix composition. The temperature to which the reinforcing material is heated is significantly higher than the temperature of the resinous matrix composition at which the impregnation takes place. The high temperature of the reinforcing fiber allows the dwell time in the resin bath to be much shorter, and the rate of production of the prepreg material to be much faster, as compared to previously known methods. The impregnated roving or tow that is produced according to the present inventive process has substantially no voids and can therefore be quickly and easily formed into a desired article having no voids or essentially no voids without the lengthy consolidation processes necessary for prepregs formed by other processes. In other words, the roving bundle is fully, or substantially fully, wet out in the prepreg of the invention. The only process that must take place in forming an article is fusion between impregnated bundles, and it is possible to use temperatures, pressures, and/or times during such forming operations that are significantly reduced over prior art processes.
The present invention also provides a method of making a molded article using the prepreg of the present invention.